1. Field of the Invention
This invention relates generally to deflection yokes and, more particularly, is directed to an improved deflection yoke for use in connection with an in-line, three-beam type color cathode-ray tube.
2. Description of the Prior Art
In a deflection yoke for a color cathode-ray tube of the in-line, three-beam type, the horizontal deflection magnetic field is made to be of the so-called pin cushion-type so as to be convergence free. However, due to the intensity distribution of such horizontal deflection magnetic field of the pin cushion-type, the outer or side beams, that is, the red and blue beams, are deflected toward the side portions of the screen or face panel by amounts that are different from the amount of deflection of the center or green beam, and the described phenomenon is usually referred to as horizontal center raster (HCR) misconvergence. More specifically, when a pin cushion-shaped horizontal deflection magnetic field is applied to the red R, green G and blue B electron beams of an in-line, three-beam color cathode-ray tube, as shown on FIG. 1A, the intensity distribution of the pin cushion-type magnetic field is as shown on FIG. 1B. Therefore, if the three beams are made coincident at the center of a picture screen SC (FIG. 2) and then deflected from the center in the horizontal direction, the amount of deflection of the side beams, that is, the red beam R and the blue beam B will be progressively greater than the amount of deflection of the center or green beam G as the extent of the horizontal deflection from the center of the screen is increased. As a result of the foregoing, the side beams R and B are displaced to positions spaced laterally outward from the center beam G, as indicated on FIG. 2, with the result that a color shift occurs in the reproduced picture.
In order to avoid the above problem, conventional deflection yokes have been constructed so that the horizontal deflection magnetic field provided thereby varies in the longitudinal direction of the yoke, that is, in the direction of travel of the electron beams therethrough so as to form a barrel-shaped magnetic field at the end or side of the yoke facing toward the electron gun or guns, and a pin cushion-shaped magnetic field at the end or side of the deflection yoke facing toward the screen or panel of the cathode-ray tube, thereby to suppress relative variations in the amounts of horizontal deflection of the red, green and blue beams for avoiding the aforementioned HCR misconvergence. However, designing the distribution of the magnetic field in the longitudinal direction so that it has a barrel-shaped configuration at one end of the deflection yoke and a pin cushion-shaped configuration at the other end does not provide a full solution to the problem in that there is no possibility to effect the further fine adjustment of such distribution after the shape of the horizontal deflection coil has been determined. In other words, due to variations in the horizontal deflection coils, as actually produced, there may be either insufficient correction or over-correction for avoiding the HCR misconvergence. This may give rise to serious difficulties, particularly, in the case of a high resolution television monitor or the like which requires final fine adjustments for obtaining sufficient accuracy in the landing positions of the electron beams.
It has also been thought to avoid or compensate for the HCR misconvergence by providing an auxiliary deflection coil supplied with a suitably adjusted horizontal deflection current from an external circuit therefor in order to generate a compensating magnetic field. However, such auxiliary deflection coil and the external circuit for supplying a suitably adjusted horizontal deflection current thereto undesirably increase the complexity and costs of the color cathode-ray tube.